When I instructed, I occasionally had students fly with a pen or pencil between their fingers so they couldn't get a death grip on the yoke.

It's a bit hard to describe, but if you can imagine the pencil oriented parallel to the first row of knuckles (perpendicular to the fingers), with the middle and ring finger gripping the yoke and the pencil on top of those two fingers. The index and little finger holding the pencil in place between the first and second knuckles. When you start to tense up, the pencil presses hard into the backs of your middle and ring finger and hurts immediately. This immediate feedback gets your attention and teaches you to relax and not try and grip the yoke so hard (otherwise, you aren't aware you're doing it until your hand hurts, as you've already discovered). Try it, it just might help you!